The present invention relates to a method and a device for the detection of targets for a wideband, unambiguous pulse Doppler radar with low frequency of repetition and high distance resolution.
In modern surveillance radars in particular, the increase in their range obliges them to adapt usually to ambiguities in distance or speed. In distance; the ambiguities result from lack of knowledge of the transmission pulse producing an echo at a given point in time. The distance ambiguity is given by D.sub.a =c Tr/2 where c is the velocity of light and Tr is the period of repetition of the pulses. The distance from a target is therefore known only to within the nearest multiple of D.sub.a. In speed, the ambiguities arise out of the very principle of the measurement of the Doppler effect, based on the measurement of the progress of the phase shift between the local oscillator and the received echo, from one pulse to the next. This progress of the phase shift measures the movement of the target between two successive pulses, assessed in wavelengths: since the phase shift is measured only to the nearest 2.pi., the speed is measured only within V.sub.a =.lambda./(2 Tr), V.sub.a giving the ambiguity in speed.
This leads to the existence of blind speeds, every V.sub.a starting from the zero speed. For a ground radar, this leads to the elimination of all the possible targets moving close to these ambiguous speeds at the same time as the clutter, and therefore limits the domain of coverage (in speed) of the radar.
For a radar of sufficient range, these problems of ambiguities become large enough for an attempt to be made to cope with them by the transmission, in each direction of observation, of several bursts of pulses with different repetition frequencies (hence blind speeds). Unfortunately, this makes it necessary, in order to maintain a sufficient rate of renewal, to reduce the duration of each burst and hence lower the resolution in speed. It is possible to obtain a situation where the number of pulses in each burst is so small that anti-clutter Doppler filtering eliminates a major proportion of the targets.
To overcome these drawbacks, it becomes necessary to conceive of radars that are ambiguous both in distance and in speed: these are Mean Frequency of Repetition (MFR) radars. These radars are moderately ambiguous in distance and in speed (they show a few ambiguities) and it becomes possible to reconstitute the real distance and speed by transmitting a small number of coherent bursts in each direction, at the cost however of a certain degree of complexity due to the ambiguity removal processing operations and a generally acceptable reduction in the resolution in speed.
In the case of airborne nose-cone radars, where the clutter echoes received have radial speeds ranging from -Vp to +Vp (Vp being the speed of the carrier), the radars have several modes of operation, again because of problems of ambiguity. These are the High Frequency of Repetition mode (HFR operation, unambiguous in speed) for the detection of targets in approach mode, the Low Frequency of Repetition mode (LFR operation, unambiguous in distance) for upward sighting and the Mean Frequency of Repetition mode (MFR operation, ambiguous in distance and speed) for the detection and tracking of targets moving away. The removal of ambiguity is done according to the multi-burst principle described here above. In general, the values of resolution in distance of these radars is in the range of about a hundred meters in watching mode.
However, resolution in distance of this kind is quite insufficient for certain applications such as cartography by airborne radar using the synthetic antenna method or the method of threat analysis (raid analysis and target classification). The high values of resolution in distance that are needed extend from some meters to some decimeters. They are generally obtained by pulse compression or by synthetic bands (the transmission of successive coherent bursts or bursts interlaced at different carrier frequencies).
These modes with high distance resolution generally avoid the problems of ambiguity by the fact that they are limited to certain conditions of operation. In cartography, the radar used is of the LFR type, and the speed domain is limited to the unambiguous domain. In threat analysis, the target is designated in advance, generally by another radar mode, and only the distance-speed domain corresponding to this designation is analyzed.
However, it is clear that a radar with high resolution in distance that is, at the same time, unambiguous, would be of very great value. One of the problems that emerge immediately, is that of the migration of the targets in distance. Indeed, owing the high resolution in distance, hence the small width of the range bins, a moving target does not remain in the same range bin between the first repetition and the last repetition of a period of analysis (burst of pulses). This does not enable the application of standard Doppler analysis, range bin by range bin.